Reflectors are often used in optical displays, such as liquid crystal displays, to even out illumination across the display and to diffusely reflect the light from the backlight or light coming in the display from the environment. Prior art reflectors include planar specular reflectors and planar diffusive reflectors. Specular reflectors include a substantially planar surface that is covered with a reflective metallic coating. Specular reflectors are characterized by an angle of incidence being substantially equal to an angle of reflection. Diffusive reflectors typically have a roughened surface which is predominately coated with a metallic reflective coating. Diffusive reflectors are characterized by reflecting and scattering incident light.
The transmission-type LCD includes a backlight, typically one to two cold fluorescent bulbs at the back surface of the liquid-crystal panel. The backlight consumes a relatively large amount of power. The diffuse reflector evens out the illumination of the back light across the entire display to eliminate any hot (brighter) spots. The more efficient a reflector is at diffusing, the more even the light will be across the display. The more efficient the reflector is, minimizing the amount of light lost to transmission and absorption, the more light passes through the liquid crystal and is realized as a brighter screen. This more efficient diffuse reflector can be used for a brighter display or for less power use that extends battery life
Reflection-type LCDs include a reflector for reflecting light at a back surface of the LCD, in which ambient light is reflected on the reflector to display images. The diffuse reflector reflects and diffuses the ambient light to mask any hot spots due to differences in ambient lighting across the display. The more efficient the reflection and diffusion of the reflector is, the brighter the display can be. This display does not use a backlight reducing the amount of required power but the ambient light reflection cannot produce satisfactory brightness for vivid color images and is used on calculator and other text displays.
A transflection display is a combination of a transmission display and a reflection display. A cell phone and PDA (personal desk assistant) are examples of this type of device. They work in both reflection and transmission mode using ambient light and a backlight alternatively. The diffuse reflector in this application reflects and diffuses the backlight and the ambient light to give a bright, even display in both modes. A transflector increases efficiency and brightness under both ambient and supplemental lighting conditions in visual display applications. In an attempt to overcome the above described drawbacks of reflective and transmissive displays, some electronic displays have been designed to use ambient light when available and back lighting only when necessary. This dual function of reflection and transmission leads to the designation, “transflective”. One problem with currently available transflective displays is that they have good performance in either reflective or transmissive mode, but not both. This stems from the fact that the backlight assembly is not as efficient a diffuser/reflector as the back reflector and diffuser traditionally used in a purely reflective display, and the display thus appears less bright when viewed under ambient light. In addition, many devices with small display screens, such as pagers, use reflective LCDs with a supplemental electroluminescent backlight for low ambient light conditions. The LCD is backed with a plastic film that is partially reflective and partially transmitting. A transflective display can be made by placing a transflective film between the rear polarizer and the backlight. The transflective film provides a specified trade-off between reflectivity for ambient lighting and transmission for backlit operation.
It is desirable to have the amount of diffuse reflectance vary across the reflection film, to compensate for uneven brightness across a backlit display. A reflection film with uniform diffuse reflection across the film must have the reflection efficiency to diffuse the most intense, specular areas of the display across the display. These reflectors tend to need high levels of reflection efficiency causing light to be scattered at a wide angle across the entire film, where the light is scattered around the edges of the film and lost. With a reflector with a variable reflection efficiency, the areas of high specular light could be more diffusely reflected than areas of less specular light. The result would be a display that had even diffuse light across it while having a higher overall reflection value compared to the uniform diffuse reflection film and a brighter display.
The variable reflector of the present invention can replace the dot printing on the light guide in an LCD. The light guide is typically a thick (approx. half a centimeter) piece of acrylic designed to guide the light from the light sources (located on at least one of the edges of the light guide) out to the display at a normal and to even the illumination from the light sources across the display. The evening of illumination is produced by a dot pattern printed on the back side (the side facing the reflector) of the light guide. The dot pattern varies in size across the display (smaller and fewer dots towards the light source and larger and more frequent dots away from the light source). The dot pattern's purpose is to try to direct more light out from the light guide away from the light sources and less light out of the display near the light sources. This causes the brightness of the display to be more homogeneous. In this prior art method of evening illumination, printing is a very costly and time consuming because each light guide is screen printed individually. The current invention of a variable reflector can produce the same result of evening out the illumination by having more diffuse areas near the light source(s) and specular areas more away from the light source(s). In addition, the current invention is a roll to roll process making it much cheaper and faster to manufacturing. Having a variable reflector with a diffuse reflectance gradient behind the light guide eliminates the need for the screen printed dots thus eliminating a processing step and saving manufacturing time and money.
Diffuse reflectors for light have been manufactured in a number of ways. Generally, diffuse reflectors are made by taking a reflective surface and roughening one of its faces. One method of manufacture involves sprinkling powders on a flat surface and gluing the powders to the surface. A second method involves grinding or blasting a metal or glass surface to achieve the necessary roughness for diffusely reflecting infrared wavelengths. A third method is to dimple an aluminum surface with a regular hexagonal array of approximately {fraction (1/64)}-inch diameter holes.
The primary disadvantages of the above methods of roughening a reflective surface is that they either do not make the surface rough enough or they do not make the roughness random enough to enable the surface to function as an isotropic diffuse reflector. If the surface is not rough enough the reflectance will not be perfectly diffuse and it will have an enhancement or peak in the specular direction that gets longer at longer wavelengths. If the roughness is non-random, the non-randomness will create diffraction effects that favor particular off-specular directions of reflection, thus making the diffuse reflectance non-isotropic. Other general methods for roughening a surface include electric discharge machining (EDM). U.S. Pat. No. 3,754,873 (Bills et al.) discloses a cold rolled sheet having a roughened surface formed by projections of such shape and arrangement that the visual appearance of the surface of the sheet is relatively constant. EDM is cost and time prohibitive to make diffuse reflectors on a large scale.
U.S. Pat. No. 5,976,686 (Kaytor et al.) relates to a diffuse reflector made of porous polymeric sheets using thermally induced phase separation technology (TIPS). The TIPS diffuse reflector can not deliver as high a reflectivity as a metallized surface. To achieve the same brightness of a backlit display with a metallized surface, a display with a TIPS diffuse reflector would have to increase the brightness of the backlight, reducing the lifetime of the battery. The light scattering regions of the TIPS diffuse reflector are on the order of the wavelength of light and could add color to the light diffused thus imparting a non-desirable coloration to the display.
U.S. Pat. No. 5,917,567 (Oh at al.) relates to a reflector having diffusion characteristics in which the surface of the reflector is formed with a plurality of convex portions by uniformly depositing fine spacers. The reflector is manufactured by providing a substrate, forming a thin layer of a solution of beads and polymer on the substrate, and forming a reflective layer on the thin layer. The beads form simple reflective lenses as compared to the complex lenses used in this invention. Complex lenses provide more efficient diffusion because of the multitude of lens surfaces and thus provide more efficient diffusion than can be obtained with a simple lens diffuse reflector.
Other diffuse reflectors used as reflectors in displays use a voided polymer structure with titanium dioxide. This provides for a high amount of diffusion, but does not have the high amount of reflectivity leading to a darker display.
U.S. Pat. No. 6,261,994 (Bourdelais et al.) describes a reflective photographic base materials made up of layers of biaxially oriented polyolefin sheet with voiding, TiO2 and colorants adjusted to provide optimum reflection properties. Voided films with TiO2 typically have diffuse reflectance measurements of 85 to 88% at 500 nm making them inferior to the variable diffuse reflectors without inorganics that have more efficient diffuse reflectances. Also, voided films tend to be thicker and therefore add weight to the display device.
U.S. Pat. No. 6,018,379 (Mizobata) describes a conventional reflective liquid crystal display that has been configured to form a convex-concave at the reflecting surface of the reflector. To form the concave-convex surface, it is necessary to deposit an insulating film and to pattern the deposited insulating film to form the convex-concave surface. The fine control of a shape such as an inclined angle of the convex-concave is difficult, with the result that a sufficient light scattering cannot be obtained. Abrading or grinding the surface with abrasive powder and further etching it with a hydrofluoric acid if necessary can also form the convex-concave surface. A light scattering coating can be formed by spin-coating. These methods described are labor and time intensive, use hazardous materials, and must be made in a sheet as instead of rolls making them prohibitively expensive.
It is known to produce polymeric film having a resin coated on one surface thereof with the resin having a surface texture. This kind of polymeric film is made by a thermoplastic embossing process in which raw (uncoated) polymeric film is coated with a molten resin, such as polyethylene. The polymeric film with the molten resin thereon is brought into contact with a chill roller having a surface pattern. Chilled water is pumped through the roller to extract heat from the resin, causing it to solidify and adhere to the polymeric film. During this process the surface texture on the chill roller's surface is embossed into the resin coated polymeric film. Thus, the surface pattern on the chill roller is critical to the surface produced in the resin on the coated polymeric film.
One known prior process for preparing chill rollers involves creating a main surface pattern using a mechanical engraving process. The engraving process has many limitations including misalignment causing tool lines in the surface, high price, and lengthy processing. Accordingly, it is desirable to not use mechanical engraving to manufacture chill rollers.
The U.S. Pat. No. 6,285,001 (Fleming et al) relates to an exposure process using excimer laser ablation of substrates to improve the uniformity of repeating microstructures on an ablated substrate or to create three-dimensional microstructures on an ablated substrate. This method is difficult to apply to create a master chill roll to manufacture complex random three-dimensional structures and is also cost prohibitive.
In U.S. Pat. No. 6,124,974 (Burger) the substrates are made with lithographic processes. This lithography process is repeated for successive photomasks to generate a three-dimensional relief structure corresponding to the desired lenslet. This procedure to form a master to create three-dimensional features into a plastic film is time consuming and cost prohibitive.
In U.S. Pat. No. 5,223,383 photographic elements containing reflective or diffusely transmissive supports are disclosed. While the materials and methods disclosed in this patent are suitable for reflective photographic products, the % light energy transmission (less than 40%) is not suitable for liquid crystal display as % light transmission less than 40% would unacceptable reduce the brightness of the LC device.
In U.S. Pat. No. 6,266,476 (Shie et al.) a monolithic element having a substrate body and a macro-optical characteristic produced by surface micro-structures. These micro-structures can be non-uniform across the lens to minimize certain lens aberrations. These non-uniform micro-structures reduce lens aberrations, but are not able to significantly alter the macro-optical characteristics of the optical body. The diffusing structures, in this invention, vary as to change the macro diffusion efficiency across the diffusion film. The diffusion elements can vary changing the diffusion characteristics of the diffusion area from diffusing most of the light to letting light pass specularly which micro-structures are unable to do.
There remains a need for an improved diffuse light reflection of image illumination light sources to provide a desired level of both light reflection and light diffusion.